1. Field of Invention
This invention relates to an internally toothed member suitable for use, for example, as a drum of a clutch in an automatic transmission, and to a method and an apparatus for forming the same.
2. Related Art
In automatic transmissions, planetary gear units are used. A planetary gear unit is made up of a sun gear, pinion gears, a ring gear and a carrier. The planetary gear unit is constructed so that gear-changing can be effected by selectively controlling the sun gear, the ring gear and the carrier using frictional engagement elements such as clutches and brakes (see Japanese Unexamined Patent Publication No. H.4-175576).
FIG. 2 shows a section of a main part of a conventional clutch. In FIG. 2, C2 is a second clutch that can be engaged and disengaged, allowing an input shaft (not shown) and a sun gear (not shown) to be selectively coupled. For this purpose, the second clutch C2 includes a rotatably disposed drum 20, which is an internally toothed member, an inner side rotating member (not shown) mounted rotatably relative to the drum 20, a plurality of outer side thin plates 22 having their outer peripheries spline-interlocked with the drum 20, a plurality of inner side thin plates 23 having their inner peripheries spline-interlocked with the inner side rotating member, a flange 19 having its inner periphery spline-interlocked with the sun gear, and a hydraulic servo C-2 for engaging the second clutch C2 by pressing together the outer side thin plates 22 and the inner side thin plates 23.
To interlock the outer peripheries of the outer side thin plates 22 and the flange 19 with the drum 20, interlocking grooves 31 extending in the axial direction are formed in the drum 20 in a plurality of locations spaced in the circumferential direction, and a plurality of teeth 22a, 19a are formed projecting radially outward from the outer peripheries of the outer side thin plates 22 and the flange 19 in a plurality of locations spaced in the circumferential direction in correspondence with the interlocking grooves 31.
The hydraulic servo C-2 is mounted to advance and retract inside the drum 20 and has a piston 25 forming a hydraulic chamber 26 between itself and the drum 20. When the piston 25 is advanced (moved to the left in FIG. 2) by hydraulic fluid being supplied into the hydraulic chamber 26, the outer side thin plates 22 and the inner side thin plates 23 are pressed together and the second clutch C2 is thereby engaged. As a result, the input shaft and the sun gear are coupled and rotation transmitted from the input shaft to the inner side thin plates 23 by way of the inner side rotating member is transmitted to the outer side thin plates 22 and further transmitted by way of the drum 20 and the flange 19 to the sun gear.
When on the other hand the piston 25 is retracted (moved to the right in the figure) by hydraulic fluid being drained from the hydraulic chamber 26, the outer side thin plates 22 and the inner side thin plates 23 are moved apart and the second clutch C2 is released. As a result, the input shaft and the sun gear are disconnected and rotation transmitted to the inner side thin plates 23 by way of the inner side rotating member is not transmitted to the outer side thin plates 22.
When the outer side thin plates 22 and the inner side thin plates 23 are pressed together as the piston 25 advances, the flange 19 tends to be advanced together with the outer side thin plates 22 and the inner side thin plates 23. To limit movement of the flange 19 toward the left in FIG. 2 and exert a reaction force on the outer side thin plates 22, the inner side thin plates 23 and the flange 19, an annular snap ring 33 is provided at the inner circumferential surface of the drum 20. For this purpose, an annular groove 20a is formed adjacent to the flange 19 in the inner circumferential surface of the drum 20, and the snap ring 33 is fitted in this groove 20a. As a result, the front end face of the flange 19 (the left end face in FIG. 2) and the rear end face (the right end face in FIG. 2) of the snap ring 33 are made to abut upon each other and movement of the flange 19 to the left in the figure is thereby limited.
Also, the sun gear meshes with pinion gears (not shown), and it sometimes happens that a thrust force generated along with rotation of the sun gear acts on the flange 19 in the direction of the arrow B. At this time, if the flange 19 were to move to the right in FIG. 2 under this thrust force, the second clutch C2 would engage, notwithstanding that hydraulic fluid is not being supplied to the hydraulic chamber 26.
For this reason, to limit movement of the flange 19 to the right in FIG. 2 and exert a reaction force on the flange 19, interlocking grooves 31 are provided with wide parts 31a and narrow parts 31b. A step 34 is thereby formed adjacent to the flange 19. The teeth 19a of the flange 19 are inserted into the wide parts 31a of grooves 31, while the teeth 22a of the outer side thin plates 22 are inserted into the narrow parts 31b of grooves 31. As a result, the rear end face (the right end face in FIG. 2) of the flange 19 abuts on the step 34 and movement of the flange 19 to the right in FIG. 2 is thereby limited. The tooth width of the teeth 19a is made wider than the tooth width of the teeth 22a so that the teeth 19a can be inserted into the wide parts 31a and the teeth 22a can be inserted into the narrow parts 31b.
A first brake (not shown) for fixing the sun gear is disposed outward of the second clutch C2 in the axial direction. This first brake consists of a band brake. The band brake is applied by a belt (not shown) being tightened, thus stopping the rotation of drum 20 and the flange 19.
However, in the conventional drum described above, as rotation transmitted to the drum 20 is transmitted to the flange 19, stress acts at the root portions of the interlocking grooves 31, and the rigidity of the drum 20 is reduced.
For this reason, a cylindrical member 36 has been fixed by welding to the outer periphery of the drum 20 to increase the rigidity of the drum 20. However, the work of manufacturing the cylindrical member 36 and also the work of fixing the cylindrical member 36 by welding becomes necessary as a result, and the increased working man-hours raises the cost of the drum 20.
Furthermore, when fixing of the flange 19 is effected by means of two snap rings, because it is necessary to form two grooves for fitting the snap rings, working man-hours for cutting and deburring machining and so on increase the cost of the drum 20.